1. Field of the Invention
The present invention relates to a coating/developing apparatus and a substrate transfer method, in which a resist film is formed by applying a resist liquid to a substrate, and is then subjected to a light exposure process by a light exposure apparatus and a subsequent developing process. Particularly, the present invention relates to a coating/developing apparatus and a substrate transfer method preferably used for an immersion light exposure apparatus for performing light exposure while immersing the resist film in a liquid.
2. Description of the Related Art
In the process of manufacturing semiconductor devices, photolithography techniques are used for forming circuit patterns on semiconductor wafers (which will be simply referred to as “wafer”, hereinafter). Where a circuit pattern is formed by use of photolithography, the process steps are performed, as follows. Specifically, a resist liquid is first applied to a wafer to form a resist film. Then, the resist film is irradiated with light to perform light exposure on the resist film in accordance with the circuit pattern. Then, the resist film is subjected to a developing process. Such photolithography steps are performed in a system including a coating/developing apparatus for performing a series of processes, such as resist coating and development after light exposure, combined with a light exposure apparatus.
In recent years, since patterns are increasingly miniaturized, resists of the chemical amplification type are used, because they have higher sensitivity. Where a resist of the chemical amplification type is used, a heating process is performed thereon by a post-exposure baking unit (PEB) after light exposure to promote the reaction. However, since the reaction proceeds in a time period from the light exposure to the heating process, if the time period from the end of the light exposure to the start of the heating process performed by the post-exposure baking unit (PEB) is fluctuated, the uniformity of line width becomes poor. Accordingly, this time period should be constant. In this respect, an interface section of the coating/developing apparatus relative to the light exposure apparatus includes a transfer arm to transfer semiconductor wafers. The transfer arm is used not only for transfer from the light exposure apparatus into the post-exposure baking unit (PEB), but also for transfer in other various steps. Consequently, the time period from the end of the light exposure to the start of the heating process performed by the post-exposure baking unit is inevitably fluctuated among wafers depending on the situation of the transfer arm.
As a technique for preventing this problem, Jpn. Pat. Appln. KOKAI Publication No. 2004-193597 discloses a technique for setting a post-exposure delay time, which is defined by the time period from the end of a light exposure performed by a light exposure apparatus to the start of a heating process performed by a post-exposure baking unit, to be constant among wafers. For this purpose, each wafer is set on standby in an unheated state inside the post-exposure baking unit (PEB) during a waiting time Tt expressed by Tt=Tmax+Tmin−Tr, where Tmax is the maximum value of a transfer start delay time, Tmin is the shortest time thereof, and Tr is the actual transfer time. With this operation, the time period from the end of the light exposure performed by the light exposure apparatus to the start of the heating process performed by the post-exposure baking unit is made constant among all the wafers.
In recent years, the integration degree of semiconductor devices becomes increasingly higher to improve the operation speed and so forth. Accordingly, photolithography techniques are required to increase the miniaturization level of circuit patterns formed on wafers. As a photolithography technique for realizing a high resolution of a 45-nm node level, there has been proposed the following immersion light exposure (for example, see U.S. Patent Application Publication No. US 2006/0231206 A1). In this immersion light exposure, a light exposure liquid, such as purified water, having a refractive index higher than air is supplied between the wafer and light exposure projection lens. The wavelength of light radiated from the projection lens is shortened by means of the refractive index of the light exposure liquid, so that the line width obtained by the light exposure is decreased.
In the immersion light exposure, the wafer is unloaded from the light exposure apparatus in a wet state, and this wet wafer is returned to the coating/developing apparatus. Accordingly, in order to prevent problems, such as formation of water marks on wafers, from being caused, a cleaning/drying process needs to be performed in the interface section.
Where an immersion light exposure is performed, therefore, a cleaning/drying process time is added to the transfer waiting time described above in the interface section, from the end of light exposure to the start of the post-exposure baking, resulting in that the maximum value of the transfer start delay time is very large.
As described above, where an immersion light exposure process is performed by the method disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2004-193597 described above, and the post-exposure delay time is made constant among wafers, the waiting time of a wafer in the post-exposure baking unit (PEB) is increased. In this case, in order to maintain the same throughput, the number of post-exposure baking units (PEB) in the coating/developing apparatus needs to be larger. However, since post-exposure baking units (PEB) are expensive, a preferable method should be arranged such that the time period until the post-exposure baking process can be made constant without increasing the necessary number of units.